"Nanoparticles," which are defined as particles with diameters of about 100 nm or less, are technologically significant, since they are utilized to fabricate structures, coatings, and devices that have novel and useful properties due to the very small dimensions of their particulate constituents. One method for synthesis of nanoparticles of a wide variety of inorganic compositions is so-called "microemulsion-mediated" synthesis.
A microemulsion is defined as a thermodynamically stable, optically isotropic dispersion of two immiscible liquids consisting of nano-size domains of one or both liquids in the other, stabilized by an interfacial film of surface-active molecules. Microemulsions usually are classified as either water-in-oil (w/o) or oil-in-water (o/w) depending on which is the dispersed phase. More generally, microemulsions of two non-aqueous liquids of differing polarity with negligible mutual solubility can also be prepared. In addition, certain systems that contain comparable amounts of two immiscible fluids will have a bi-continuous structure. When a dispersed phase is present, it will consist of monodispersed droplets, usually in the size range of 2 to 50 nm. These droplets are also known as micelles; in water-in-oil microemulsions, the aqueous droplets are also known as reverse micelles.
The nature of the surface-active film is key to microemulsion formation. By selection of appropriate surfactant chemistry and use of relatively large amounts of surfactant, microemulsions are produced spontaneously (i.e., without need for significant mechanical agitation). The very fine dispersed spherical micelles in microemulsions are thermodynamically stable due to a combination of very low interfacial tension (10.sup.-2 to 10.sup.-3 mN/m) and a significant entropy of mixing from the very large numbers of particles produced. Two general types of surfactant systems are used to produce microemulsions. Most microemulsions utilize a two-surfactant system. For example, an oil can be emulsified first in water using an aqueous surfactant (or soap) such as potassium oleate. Addition of a co-surfactant, generally an alcohol of intermediate chain length, causes the milky emulsion to clear spontaneously due to formation of very small spheres of dispersed oil. Physically, this occurs by the alcohol molecules diffusing to the oil/water interfaces, where they orient in ordered fashion between the ionized soap molecules. This reduces the interfacial tension between the phases to near zero and allows appreciable bending of the surface film, thereby producing very small particles of dispersed phase. Use of relatively large amounts of surfactant and co-surfactant in these systems is necessary, since the relative amount of interfacial area in microemulsion systems per unit volume is intrinsically very high.
Certain non-ionic surfactants also can form microemulsions, often without need for co-surfactants, usually simplifying the phase chemistry of such systems. Example surfactants in this case include polyoxyethylene-nonylphenol ethers. The amphipathic structure of such surfactants allows them to form films with very low interfacial tension between oily and aqueous phases. The phase chemistry of these systems may exhibit quite strong temperature-dependent behavior in comparison to two-surfactant systems.
In general, an appropriate surfactant system will produce both oil-in-water microemulsions and water-in-oil microemulsions; only the exact composition of the system determines which type forms. Bi-continuous phase structures are usually formed when the oil and water contents of the system are closely matched. Furthermore, a number of ordered liquid crystalline phase structures and mixtures of different phases occur in most microemulsion systems.
The properties of water-in-oil microemulsions have been exploited for highly controlled formation of nanoparticles having a very homogeneous size distribution. In w/o microemulsions, the reverse micelles continuously collide, coalesce, and break apart due to Brownian motion, resulting in a continuous exchange of their contents. Solute initially present in a limited number of droplets will eventually be present in all droplets at an equilibrated average concentration. The kinetics of the collision process depend upon the diffusion of the aqueous droplets in the continuous oily phase, while those of the exchange process depend on parameters such as the attractive interactions between surfactant "tails" exposed to the oil phase and the rigidity of the interface as aqueous droplets approach close to each other. Use of microemulsions as "nano-reactors" for ultra-fine particle formation begins by dissolving a water-soluble reactant in a w/o microemulsion and allowing it to reach equilibrium distribution in each micelle. From this point, there are three general methods used to form nanoparticles within the aqueous micelles. These are illustrated in attached FIG. 1 as(a), (b) and (c)..sup.1 FNT .sup.1 V. Pillai et al., Adv. In Coll. And Interface Sci., 55, 241 (1995).
In the first technique, two identical water-in-oil microemulsions are first formed. Subsequently, reactant species A is added to one microemulsion while reactant species B is added to the other. A and B are soluble in the aqueous micelles and may be solid, liquid, or vapor species. A and B are chosen such that the soluble cationic portion of one reactant and the soluble anionic portion of the other reactant in the aqueous phase react to form a product of extremely low solubility, thereby resulting in product precipitation.
After allowing for the distribution of the dissolved species to equilibrate in the individual microemulsions (generally this occurs very quickly, e.g., within a few seconds to no more than several minutes), the two microemulsions are mixed. Due to collision and coalescence of the droplets, the reactants A and B come in contact with each other and react to form nano-sized precipitates. This precipitate is confined to the interior of the microemulsion droplets and the shape of the particle formed reflects the interior of the droplet. In this process, the overall reaction rate is controlled by the rate of coalescence of droplets if the intrinsic chemical reaction rate is fast. Because of this, properties of the microemulsion interface such as interfacial rigidity, as determined by specific surfactant chemistry, can strongly influence the measured reaction rates. Relatively rigid interfaces decrease the rates of droplet coalescence and overall reaction, while substantially fluid interfaces enhance these rates. The properties of the continuous oil phase and ionic strength and pH of the aqueous phase also can be manipulated to control reaction kinetics. Therefore, this reaction method provides a great degree of control over the nanoparticle formation process.
The other methods for nanoparticle synthesis via microemulsions are modifications to the first method and utilize only a single microemulsion containing a dissolved reactant. One of these involves subsequent addition of a reducing agent in the form of a liquid or gas (e.g., hydrogen or hydrazine) to the microemulsion, while the other involves addition of a precipitating agent, also as a liquid or gas (e.g., O.sub.2, CO.sub.2, NH.sub.3). The former method has been used for production of metallic particles, while the latter has been employed for formation of nanoparticulate oxides, oxide precursors and carbonates.
Of importance is the fact that, for all of these methods, although the size of the nanoparticles produced is highly controllable, it is generally not directly related to the size of the aqueous micelles and usually is not easily correlated with the amount of reactant originally present in each droplet. Even small nanoparticles (e.g., having diameters of 2 nm to 5 nm) contain from about 300 to 1000 atoms, which is in most cases appreciably larger than the number of reactant molecules present in each micelle prior to reaction. This indicates that nanoparticle nuclei first form in a small fraction of micelles; these then consume the reactant in other micelles through collision-coalescence processes. Through empirical adjustment of initial reactant concentrations and microemulsion compositional parameters, nanoparticles with homogeneous size distribution and average particle diameters ranging from about 2 nm to about 100 nm can be produced via microemulsion synthesis. In certain cases, the nucleation rate of particles can be appreciably greater than the growth rate of the particles, resulting in precipitation of very small nanoparticles (e.g. 2 nm or less in diameter) in each micelle.
Since the first use of microemulsion synthesis techniques for ultra-fine catalyst production in 1982,.sup.2 an appreciable amount of research has been performed on specific microemulsion synthesis techniques to obtain a large number of chemical compounds in nanoparticulate form. Much of this work has been performed by Shah and co-workers, who have very recently reviewed the current state of the art in this area..sup.1 Compounds that have been produced successfully with these techniques include rare metal catalysts (Pt, Pd, Rh, and Ir), nickel and iron catalysts, semiconductors (CdS, CdSe), carbonates (including calcium carbonate, an important motor oil additive), silver halides, magnetic oxides (Fe.sub.2 O.sub.3, barium ferrite), zinc oxide, and multi-cation high-temperature superconductors (Y--Ba--Cu--O and Bi--Pb--Sr--Ca--Cu--O systems). Furthermore, the product particles of these syntheses have been incorporated in devices having improved or novel properties as a result of the nanostructure of their components. FNT .sup.2 M. Boutonnet et al., Colloids Surfaces, 5, 209 (1982). FNT .sup.1 V. Pillai et al., Adv. In Coll. And Interface Sci., 55, 241 (1995).
The microemulsion synthesis technique has a number of advantages in comparison to alternative methods for formation of nanoparticles, viz:
It utilizes inexpensive and common reactant compounds (e.g., water-soluble salts). PA1 For production of liquid-dispersed nanoparticulate products, only simple and inexpensive hardware is required. PA1 It is a generic method for preparation of nanoparticles of any type of composition, including metals, oxides, borides, and semiconductors. It is also readily amenable to production of coated nanoparticles, i.e., nanocomposites. PA1 It allows for a very high degree of control over the chemical and physical characteristics and homogeneity of the particles. Very narrow-size-distribution powders are prepared and their average size can be readily tailored. For multi-cationic compositions in particular, the microemulsion process allows a degree of control of chemical homogeneity that is not attainable with nearly all other nanoparticle synthesis methods. PA1 precipitating the inorganic nanoparticles within a non-continuous micellar phase in a microemulsion containing a non-continuous micellar phase and a continuous phase PA1 concentrating the nanoparticles for recovery by ultrafiltration employing a semipermeable membrane with a pore size selected to retain substantially all the nanoparticles precipitated in the micellar phase, while permeating the microemulsion continuous phase and the micellar phase not containing precipitated nanoparticles.
Bench-scale methods developed to date for synthesis of nanoparticles via microemulsions are directly applicable to scale-up to high volume production. However, in order to be practical (cost-effective and environmentally acceptable) on a large scale, an effective processing scheme is necessary that allows both: (i) recovery of non-aggregated nanoparticles, either in a concentrated stable liquid dispersion suitable for coating applications, or as solid powder; and (ii) recycle and reuse of the oils and surfactants used in the microemulsion synthesis in order to limit costs for both process inputs and organic waste disposal. The bench-scale research on microemulsions performed to date has typically utilized centrifugation of the entire microemulsion combined with co-solvent extraction/rinsing methods for nanoparticle recovery. These techniques are not attractive for application to industrial-scale production of nanoparticles via microemulsion synthesis since they are not readily scaled up and would produce large quantities of organic waste.
Thus, successful development of an economical large-scale manufacturing process for nanoparticles based on microemulsion synthesis requires identification and development of a practical processing scheme for nanoparticle recovery and oil/surfactant recycle. Such an enabling process is the subject of this invention.